This invention relates to fault-current limiters, and more specifically relates to a novel fault-current limiter for high power electric systems in which a fault-responsive device, such as sodium fuse, controls the insertion of current-limiting impedance into this circuit under given current conditions.
The utility industry is experiencing a rapid growth in system short-circuit currents as a result of ever increasing generator sizes and system interconnections. These changes have been instituted in a quest for economies and greater reliability but often unknowingly have been offset by changes in other parts of the system. Thus, circuit breakers have become more expensive; system components have had to be increased in mechanical strength to overcome the magnetic forces created by the higher short-circuit currents. Reliability has been reduced due to the transformer through-current destructive forces. System stabililty has been affected as a result of the larger fault currents. Expenses have been incurred as older circuit breakers have had to be replaced due to the increased interrupting requirements.
A fault-current limiter which can limit fault current to between 30 and 40 percent of its potential peak value will reduce the requirements for high-current interrupting circuit breakers, reduce transformer thru-currents, and allow transformer designs which are more reliable and economical. It will also allow more economical overhead line designs as line burndown problems will be reduced and will increase system stability. In general, a fault-current limiter will reduce costs and increase system reliability wherever magnetic forces and fault current thermal problems are limitations in design. Systems operating practices (i.e., system stability) and economics of construction will be enhanced.
The present fuel shortage is going to require even larger generating units in the future through the use of overlapping transmission grids to increase reliability which will add to the already rapid growth of fault currents being experienced by the electric power utilities.
In one major utility 230 kV system, fault levels have grown from a maximum of 6 GVA in 1955 to 15 GVA in 1965 and to considerably greater than 20 GVA by 1970. At the present time, the system is operated with interconnections opened to limit fault-current levels within the capability of connected buses, disconnects, and circuit breakers. The elimination of such interconnections however jeopardizes security and stability of the system and imposes inefficiencies in operation of the system. These penalties are paid to obtain adequate life on recently installed substation equipment to avoid the heavy costs of replacement of this equipment and to keep fault levels down to levels for which equipment designs are available.
On this same system, major increases in connected generation will be made within the next few years. Fault levels in some parts of the system will increase to an excess of 30 GVA by about 1975. Projecting this curve of fault level growth, one can project need for 40 GVA by the early 1980's. Thus, equipment purchased today with 30 GVA fault capability may have a life expectancy of less than 10 years.
Power transformer failures are rapidly increasing due to exposure to higher and higher through-fault currents. High costs and penalties are being paid by allowing fault-current levels to increase unchecked as system growth takes place.
To date, however, there is only one circuit and system that has found limited application to fault-current limitation in electric power systems. This is the series L-C resonant form of limiter, in which the capacitance element is switched out under fault current conditions, leaving the inductive impedance of the inductor in the series circuit.
To date the L-C resonant limiter has been developed and applied employing component or element packaging, and saturable reactor switching of the capacitance element. Because of these factors, this form of limiter is uneconomical and is physically so large (as large as the main power transformer of the system) that substation space cannot often be found to accommodate it.